A value indicating whether (and how) tic marks are drawn for the major
tic positions of a chart-axis in a chart-widget. Here are the possible values:

nil - Major tic marks are not drawn.

t - Major tic marks are drawn, and are drawn
at the major tic positions. This is the default value.

:after - Each major tic mark is drawn midway
between its official position and the position of the next major tic
mark. This may be useful for delimiting groups of non-stacked bars,
for example, rather than pointing to the middle of each group of bars.
This choice is probably not aesthetic when draw-minor-grid-lines is true.

:before - Like :after, except
that each tic mark is drawn between its official position and that of
the previous tic mark.

:between - Like :after and
:before except that no tic mark is drawn before the
first major tic position or after the last one.